Geodynamics and some Structural Geology

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Geodynamics and some Structural Geology

• Rob Bialas

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Geodynamics

• The branch of geophysics that studies the
  processes leading to deformation of planetary
  mantle and crust and the related earthquakes and
  volcanism that shape the structure of the Earth
  and other planets.

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• FORCE (Wikipedia) an influence that may cause a
  mass to accelerate
• F m a
• NewtonA newton is the amount of force required
  to accelerate a mass of one kg at a rate of one m
  per second squared
• Kg m/ s2

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FORCES 2 types

• BODY FORCES
• No physical contact between 2 objects
• Acts through the volume of a body (eg. gravity)
• SHEAR FORCES
• Contact between 2 objects
• Surface forces

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STRESS and STRAIN

• STRESS force per unit area.
• In geology we never see stress. We only see
  the results of stress as it deforms materials.
• STRAIN the amount of deformation an object
  experiences compared to its original size and
  shape.
• e ?L/L

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Ways Material Can Behave

• Elastic
• Plastic
• Viscous
• Combinations of those

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Elasticity

• Deformation is recoverable! (sEe)
• Example (rubber band snaps back)

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Plasticity

• Deformation is pushed beyond elastic limit and is
  no longer recoverable
• Example, you stretch the rubber band until it
  breaks. The part where it breaks is
  irrecoverable (plastic)

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Elastic vs. Plastic
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Viscous behavior

• Definition viscosity t µ du/dy
• Relates velocity to shear stress
• viscosity µ is in units Pa s kg m-1 s-1
• Water running over streambed low viscosity
  low shear stress
• Mantle flowing under plates high viscosity
  (1018 to 1020) high shear stress

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Viscosity Examples
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Viscosity Example Asthenospheric Counterflow
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Things that happen on Earth that all the above is
used to describe

• Plate Flexure
• Mantle Convection
• Faulting
• Mountain Building
• Extension
• Subduction
• etc.
• (yes, some of these overlap)

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Plate Flexure
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Flexure Elastic vs Plastic
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Some real examples Hawaii
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Flexure and Earthquakes at Subduction Zones
Nelson 1996
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Strength the Christmas Tree Model
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Faulting

• 3 major kinds
• Thrust faults
• Normal Faults
• Strike Slip Faults
• Got to http//www.tinynet.com/faults.html for
  cheesy cartoons of normal, thrust, and strike
  slip.
• (Reverse fault is a thrust fault at a high angle)

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Strike Slip Faults

• Eg. San Andreas, N. Anotolian Fault
• Connection of segments, you can stress up one
  section and then another fires off
• Is the San Andreas Week? big debate
• We dont see enough heat at the SAF so
• 1. hydrothermal fluids distribute heat
  efficiently
• 2. the fault has low friction and is weak

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Compressional Regime example Coloumb Wedge
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Folds

• In the lower crust, where things can flow, folds
  can form under compression
• Anticlines A shaped oldest in middle
• Synclines U shaped youngest in middle

Small scale folding
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Folding large scale Sheep Mountain Anticline
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Extensional Systems

• Narrow Rifts (Red Sea, East African, Rio Grande
  Rift (not active))
• Wide Rifts (Northern Basin and Range, parts
  Ageaen)
• Core Complexes (Southern Basin and Range, parts
  Ageaen)

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Low Angle Normal Fault Paradox

• Andersonian Fault theory predicts faults go
  inactive at 30o,but we observe faults with a
  lower dip in the field
• No earthquake with a low angle mechanism has been
  observed (according to 99.9 of researchers
• There are 1. active at a low angle, 2. inactive
  and cut by active, high angle faults 3. They form
  at a high angle and due to strain weakening can
  be active at lower angles, but eventually get cut
  off.

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Brittle Ductile Transition

• Top part cold is brittle
• Lower crust is hot, ductile, it can flow.
• The below picture is suspect (do faults really
  cut into the ductile crust and mantle this easily

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LANF 2

• Roll over model (Lavier 1999)
• People at lamont say inactive or roll over. NCB
  and Mark Anders are huge in the anti-LANF camp.

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Subduction Zone Summary
FR force of viscous resistance, FSP slab pull
force, FSA sea anchor force, FSU suction force
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Subduction Accretion vs. Erosion
accreting margin
non-accreting margin
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Rollback (which we dont entirely understand)
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Example Scotia arc falling back into the Atlantic
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Mantle Convection

• Down flux and recycling of material at subduction
  zones
• Upward movement at plumes
• Huge area of research
• 1 or 2 layer convection? - someone else chime in
  here
• cool movies of convection at
• http//www.gps.caltech.edu/gurnis/Movies/movies-
  more.html

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Geodynamics of Other Planets Mars

• Early Mars had plate tectonics but Martian crust
  is too light and thick to subduct
• Subduction ceased, convection in mantle and core
  cease, and dynomo ceased
• Volcanoes are evidence of past plate tectonics.
  Still active
• There was also water but it is mostly gone now
  (small ice caps, some subsurface ice, some water
  lost to space, some tied up in sediments)

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MARS
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Venusaltimeter topography
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Venus latest theories

• No impact of body that made moon on earth
  happened to Venus, so it kept early atmosphere.
  Green House effect ran-away evaporated ocean,
  which inhibited subduction and no volatiles were
  recycled. This kept the Venus mantle strong and
  there is no convection. Occasionally a
  singularity developes and huge amounts of
  volcanics are expelled on the surface.

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Io
Europa

• Ice covered, but we dont know how much
• Long rifts whole length of planet but no obvious
  compressional features

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Io

• Tons of volcanoes due to tidal affects of sun,
  jupiter, and other moons.
• Predicted and published by 3 sweet dudes a few
  months before Voyager arrived.